It is generally known that an exhaust gas in a diesel engine or a lean-burn gasoline engine contains NOx or particulate matter (hereinafter, referred to as PM) including carbon as a major constituent, which is a cause of air contamination. Therefore, various systems and methods have been proposed for trapping and removing the PM from the exhaust gas.
Among the systems and methods for reducing PM, it is generally known to dispose a diesel particulate filter (hereinafter, referred to as DPF) in an exhaust passage for collecting PM with this DPF. In addition, for oxidizing and removing the collected PM, there is proposed a so-called catalytic DPF of supporting catalysts with DPF (for example, Japanese Patent Laid-Open No. 2001-207836, Japanese Patent Laid-Open No. 2003-154223, Japanese Patent Laid-Open No. 2004-92584, etc.).
The technology described in Japanese Patent Laid-Open No. 2001-207836 does not uniformly distribute catalysts in the entirety of DPF but distributes more catalysts on the upstream side of the exhaust gas flow in DPF than on the downstream side, and also distributes more catalysts at the inflow side of the exhaust gas than at the outflow side in a partition wall, thus supporting the catalysts.
In addition, the technology described in Japanese Patent Laid-Open No. 2003-154223 shows a wall-flow type particulate filter in which a predetermined amount of catalysts are supported on one end side and the catalysts are supported to reduce in amount from the one end side toward the other.
Further, the technology described in Japanese Patent Laid-Open No. 2004-92584 likewise shows a wall-flow type particulate filter in which alkali metallic catalysts which do not contain noble metals are supported on the upstream side of a collection wall and also absorption NOx catalysts are supported on the downstream side of the collection wall.
In addition, it was recently disclosed the technique of oxidizing and treating PM by using ozone O3 having a stronger oxidation effect as compared to NO2 (for example, Japanese Patent Laid-Open No. 2005-502823). In a method and a system for performing post-treatment of an exhaust gas in a diesel engine described in Japanese patent laid-Open No. 2005-502823, a device for generating ozone O3 or nitrogen dioxide NO2 as an oxidant from the exhaust gas by plasma is disposed on the upstream side from a particulate filter and is designed to oxidize and remove soot collected in the particulate filter by selectively using the ozone and the nitrogen dioxide at a low temperature and the nitrogen dioxide at a high temperature in response to a temperature of the exhaust gas.